1. Field of the Invention
The present invention relates to an image stabilization control circuit provided in an imaging apparatus (e.g., a digital still camera), which can prevent a captured image from being blurred when the imaging apparatus moves or vibrates (camera shake).
2. Description of the Related Art
Recent imaging apparatuses, such as digital still cameras and digital video cameras, are equipped with a high-resolution image sensor having a large number of pixels and capable of capturing a high-quality image. However, if a user's hand holding an imaging apparatus vibrates in a shooting operation, a captured image is blurred. Therefore, to obtain a high-quality image, it is desired that an imaging apparatus has a camera-shake correction function.
More specifically, an imaging apparatus includes a detection element (e.g., gyro sensor) that detects an angular speed caused when the imaging apparatus vibrates. The imaging apparatus drives an optical element (e.g., a lens or an image sensor) according to a detected angular speed component so as to prevent a captured image from being blurred. Thus, even when the imaging apparatus vibrates, a captured still/moving image signal does not include any vibration component and therefore the imaging apparatus can obtain high-quality still/moving image signal free from blur.
FIG. 2 is a block diagram illustrating a conventional image stabilization control circuit 100 that has a camera-shake correction function. The image stabilization control circuit 100 is provided in an imaging apparatus and operates under the control of a main control circuit (not illustrated) provided in the imaging apparatus. The image stabilization control circuit 100 is connected to a position detection element 200, a lens driving element 300, and a vibration detection element 400.
The position detection element 200 detects the position of a lens provided in the imaging apparatus. For example, the position detection element 200 is a Hall element that generates induced current according to the absolute position of the lens and outputs a voltage signal.
The lens driving element 300 moves the lens according to a lens driving signal generated by the image stabilization control circuit 100. For example, the lens driving element 300 is a voice coil motor. The image stabilization control circuit 100 adjusts a voltage value applied to the lens driving element 300 (i.e., voice coil motor) and controls the position of a movable coil in the voice coil motor. In other words, the image stabilization control circuit 100 adjusts the position of the lens. The lens driving element 300 moves the lens on a plane perpendicular to an optical axis of the imaging apparatus.
The vibration detection element 400 detects vibration occurring in the imaging apparatus and outputs a detection signal to the image stabilization control circuit 100. For example, the vibration detection element 400 is a gyro sensor that can generate an angular speed signal corresponding to the vibration occurring in the imaging apparatus and outputs the generated angular speed signal to the image stabilization control circuit 100.
It is desired to provide at least two elements for each of the position detection element 200, the lens driving element 300, and the vibration detection element 400. Two or more elements corresponding to a horizontal component and a vertical component can be positioned on a plane perpendicular to the optical axis of the imaging apparatus for lens position detection, lens movement detection, and vibration detection of the imaging apparatus.
The image stabilization control circuit 100 includes a servo circuit 120, a lens driver 140, an analog-digital converter (ADC) 142, a central processing unit (CPU) 160, and a digital-analog converter (DAC) 162.
The servo circuit 120 generates a control signal supplied to the lens driving element 300 according to a voltage signal received from the position detection element 200. The servo circuit 120 includes an analog filter circuit associated with external electrical parts (e.g., a resistance element and a capacitor). The servo circuit 120 generates a control signal supplied to the lens driving element 300 so that the optical axis of the lens accords with the center of the image sensor provided in the imaging apparatus. The lens driver 140 generates a lens driving signal supplied to the lens driving element 300 based on an output signal of the servo circuit 120.
The ADC 142 receives an angular speed signal (an analog signal) from the vibration detection element 400 and converts the input analog signal into a digital signal. The CPU 160 receives an angular speed signal (a digital signal) from the ADC 142 and generates an angular signal indicating a movement amount of the imaging apparatus based on the input signal. The CPU 160 is connected to a memory (not illustrated) and performs angular signal generation processing based on a software program stored in the memory. The DAC 162 converts an angular signal (a digital signal) generated by the CPU 160 into an analog signal.
The servo circuit 120 generates a lens driving signal supplied to the lens driving element 300 based on a sum of an analog angular signal received from the DAC 162 and a voltage signal received from the position detection element 200. In other words, to eliminate any blur on an image caused by camera shake, the servo circuit 120 changes the position of the lens based on an angular signal indicating a movement amount of the imaging apparatus and enables the image sensor to capture an object image not including any blur. Thus, the imaging apparatus can obtain high-quality still/moving image signal while suppressing blur on a captured image caused by camera shake.
The conventional image stabilization control circuit 100 illustrated in FIG. 2 receives an angular speed signal obtained by the vibration detection element 400 and generates an angular signal indicating a movement amount of an imaging apparatus. To this end, the CPU 160 executes a software program that realizes the above-described functions. In this case, the image stabilization control circuit 100 is required to quickly accomplish the processing. The CPU 160 operates with a high-speed clock. For example, when an imaging apparatus obtains a moving image consisting of 30 frames during a shooting operation of one second, the position of the lens is controlled at a resolution faster than 1/30 second.
The CPU 160, if operating with a high-speed clock, increases power consumption in the image stabilization control circuit 100. An imaging apparatus has a power source such as a secondary battery (e.g., lithium-ion battery) that drives the image stabilization control circuit 100. If power consumption in the image stabilization control circuit 100 increases, the residual capacity of the secondary battery quickly decreases and the operable time of the imaging apparatus becomes shorter. The shooting time for a moving image becomes shorter. The maximum number of still images that a user of the imaging apparatus can take becomes smaller. The camera-shake correction function of an imaging apparatus is active not only during a shooting operation of a moving image or a still image but also in a preview operation (shooting preparation). Therefore, it is desired to reduce power consumption required for the camera-shake correction function.